There has long been a serious problem in providing cobalt-based magnetic powders having good magnetic properties and good corrosion properties. Materials having good magnetic properties tend to have lower corrosion resistance than is desirable, (especially when in the micron- or sub-micron size ranges, i.e., in the making of single-domain particles). When substantial quantities of a material like nickel are used to increase the corrosion resistance, the magnetic properties of a material formed primarily of cobalt or iron markedly.
A great deal of work has been done on particulate metal powder systems in an attempt to provide powders which are economically practical in magnetic recording systems. A substantial part of this work has been directed to study of systems involving cobalt-based powders and combinations of chromium and or iron with cobalt. In general, the work has resulted in serious practical problems including batch-to-batch variations in product characteristics, and chemical stability under humid air conditions. Some such problems are discussed in the literature, e.g. by Bates and Alstead in IEEE Transactions on Magnetics, MAG-5, December 1969, Page 832. Much of this work was done on a composition having a major amount of iron. One such material (55% iron, 40% cobalt, 5% nickel) thought to be particularly useful by the investigators was not commercially successful, probably because of the aforesaid problems.
Other work included that of Luborsky. The literature indicates that particles produced by his method of electro-depositing the metal on mercury, were not stable. Luborsky used up 41.5% cobalt in his iron-cobalt systems. His materials, which also would have very low magnetic moment on a volume basis, were described in The Physics Of Magnetic Recording, by C.D. Mee (North Holland Publishing Company, Amsterdam; 1968).
Haines and Johnston, authors of U.S. Pat. Nos. 3,574,685 and 3,574,683, respectfully, have worked with iron-cobalt-nickel systems and have recommended systems of 60% iron and 40% cobalt. Although, they have claimed the capability of making an entire range of cobalt-iron-nickel materials, it appears from attempts to duplicate their work, their compositions tend to be predominantly iron, in spite of cobalt's presence in the reaction medium.
Ehrreich and Reti, in commonly-owned and co-pending U.S. Ser. No. 228,387, disclosed an 85% cobalt -- 15% nickel system which was an improvement over any known prior art materials but which lacked the advantages -- especially corrosion resistance of the material of this invention to be described below. As with Luborsky, it appears that unessential contamination of the product inherent in the techniques of the inventors, interfered with their obtaining optimum properties. Moreover, such a material is relatively expensive since it lacks any iron.
In most of the work directed to cobalt-nickel-iron alloy, materials have been produced in the area of 5 - 12% nickel but with about 50 percent of iron or more. Besides having the basic problems associated with less-than-desirable combination of corrosion resistance and magnetic properties, the manufacture of this type of material is illustrative of problems which beset thoe attempting to scale up processes for making magnetic-alloy powders, by forming the alloy in the particulate form. Normally such processes involve a reaction in liquid medium whereby the metal atoms first come together to form either an alloy particle or a precursor to the alloy particle, such precursors, say oxalate salts, contain the metal atoms which, on reduction, or some such conversion step, will from the metal particles. The problem is to assure that batch-to-batch, and even particle-to-particle, uniformity is achieved even if small variations in metal ratios are caused by inattention of workmen, less than optimum mixing, variations in raw materials, or whatever reason. In these systems even small changes in metal ratio can result in major changes in magnetic properties of the particle or batch. For example, a targeted 52:5:43 ratio of iron, nickel and cobalt respectively would yield a maximum permeability of about 1000, but if a particle had a 50:8:42 ratio, this permeability would drop to about 500. The ability of simultaneously overcoming this processing problem and the corrosion and magnetic-property problems, will be seen to be an important feature of the process disclosed below.